1. Field of the Invention
The present invention relates to a synchronous detector for detecting a synchronization signal from a composite signal including a synchronization signal, a luminance signal and a chrominance signal, a luminance signal including a synchronization signal and so on and, particularly, to a synchronous detector for detecting a synchronization signal stably.
2. Description of Related Art
A video signal contains a horizontal synchronization signal and a vertical synchronization signal so that it is reproduced correctly with a reproducing unit. The reproducing unit therefore has a synchronous detector for detecting a synchronization signal from the video signal. The synchronous detector is a critical circuit to display images stably.
For example, Japanese Unexamined Patent Application Publication No. 2000-244766, which is referred to herein as the conventional example 1, discloses a synchronous detector for detecting a horizontal synchronization signal correctly by removing a noise, chrominance signal and so on.
The synchronous detector of the conventional example 1 performs low-pass filter (LPF) processing in order to remove the noise superimposed on a video signal and detect a horizontal synchronization signal correctly. The LPF processing has the effect of reducing the noise on a horizontal synchronization portion. However, the LPF processing causes waveform distortion of a horizontal synchronization signal in the cases such as when a back porch pulse inserted in a copy guard signal exists and when a front porch period is too short for a horizontal synchronization signal. While the LPF processing reduces the amplitude of a signal superimposed on a front porch or back porch pulse, it smoothes the signal along the time axis to cause distortion of the waveform of a horizontal synchronization signal at the same time. This hinders the accurate detection of the rising or falling phase of the horizontal synchronization signal, which leads to distortion of video images.
FIG. 7 is a view to describe the drawbacks due to the LFP processing. In FIG. 7, the symbol “Din” represents an input video signal, “H” represents a horizontal blanking period, “FP” represents a front porch, and “BP” represents a back porch. The video signal Din contains a back porch pulse P1 as a copy guard signal. If the LPF processing described above is performed in this case, the result is a signal D11 shown in FIG. 7. In the signal D11 after performing the LPF processing on the video signal Din, the amplitude of the back porch pulse P1 is reduced and smoothed along the time axis and therefore the rising waveform of the horizontal synchronization signal is distorted. If the horizontal synchronization signal with the distorted signal waveform is detected at a horizontal synchronization detection level HL, the rising timing of an original horizontal synchronization signal is detected wrongly in the detected synchronization signal HS. Detecting a horizontal synchronization signal HS from such a signal D11 results in a signal having a horizontal synchronization period TH2 that is different from an original horizontal synchronization period TH1.
Since the signal waveform of the horizontal synchronization signal contained in the video signal is distorted due to removal of high frequency component by the LPF, an error occurs between the detected horizontal synchronization signal period and an actual horizontal synchronization signal period. To overcome this drawback, a technique for detecting a synchronization signal without directly performing the LPF processing is disclosed in Japanese Unexamined Patent Application Publication No. 2004-96233, which is referred to herein as the conventional example 2. FIG. 8 is a block diagram showing the configuration of a conventional synchronous separation apparatus, which is referred to hereinafter as the synchronous detector. FIG. 9 shows signals output from each block of the synchronous separation apparatus. The synchronous detector 80 described in the conventional example 2 includes a filter circuit 81, synchronous separators 82 and 83, a mask signal generator 84 and a mask circuit 85.
The filter circuit 81 receives a video signal such as SA1 shown in FIG. 9 and performs LPF processing and band pass filter (BPF) processing. It removes the high frequency component of the video signal, which is a burst signal, a chrominance signal superimposed on a luminance signal and a negative noise contained in the video signal, and outputs a signal SA2 shown in FIG. 9.
The synchronous separator 82 detects each synchronization on the signal SA2 after the filter processing at a level for detecting a horizontal synchronization signal and a vertical synchronization signal. SH2 in FIG. 9 represents a horizontal synchronization signal that is detected at a detection level HL in the synchronous separators 82.
The mask signal generator 84 generates a mask signal SM that becomes L level at a timing that is slightly earlier than a start of synchronization of the detected horizontal synchronization signal SH2, for example, and becomes H level at a timing that is slightly later than an end of synchronization. Specifically, the mask signal generator 84 detects a timing that is slightly earlier than a period TTH of the horizontal synchronization signal SH2, which is a timing (SK2) that is slightly earlier than a falling edge of the horizontal synchronization signal period TH2 and also detects a timing (SK1) that is slightly later than a rising edge of the horizontal synchronization signal period TH2. From SK1 and SK2, it generates a mask signal SM in which a period whose length is slightly longer than the horizontal synchronization signal period TH2 of the horizontal synchronization signal SH2 in the time axis direction is an unmasking period (L level period). The mask signal SM is supplied to the mask circuit 85.
The mask circuit 85 receives the mask signal SM and the video signal. The video signal passes through during the L level period of the mask signal SM while it is biased to a fixed value during the H level of the mask signal SM. The mask circuit 85 thus makes only a horizontal synchronization signal or a vertical synchronization signal pass through during the L level period of the mask signal SM. The synchronous separator 83 receives the signal composed of only the horizontal synchronization signal and the vertical synchronization signal from the mask circuit 85, extracts the horizontal synchronization signal SH3 and the vertical synchronization signal SV3 and outputs them.
The above processing allows detection of the horizontal synchronization signal and the vertical synchronization signal without performing the LPF processing on the input video signal.
The problem that jitter is generated in the detected horizontal synchronization signal due to distortion of the signal waveform of the horizontal synchronization signal as shown in FIG. 7 occurs normally when using the LPF whose cutoff frequency is about 500 KHz. Since the horizontal synchronization signal is 15,734 KHz, using such a LPF causes the falling and rising edges of the horizontal synchronization signal to be converted to a curved waveform. Thus, besides when a back porch pulse exists in a back porch as in the synchronous detector of the conventional example 1, in the cases such as when a front porch period is too short due to distortion of the horizontal synchronization signal and when large noise exists, performing the LPF processing to reduce the amplitude of each signal superimposed on a standard signal and noise to smooth it in the time axis direction affects the waveform of the horizontal synchronization signal and changes falling and rising timings. Hence, merely performing the LPF processing causes jitter and therefore unable to solve the problem if there is a horizontal blanking period where a signal or the like which is different from the standard signal is included.
On the other hand, if a synchronization signal is detected without performing the LPF processing as in the synchronous detector of the conventional example 2, a high frequency noise on the video signal causes error in detection of falling and rising edge of a horizontal synchronization signal particularly in the case of a VTR signal after repeated recording and a TV signal with a low electric field, thereby increasing jitter.
Therefore, it is possible to detect a synchronization signal accurately if the LPF processing can be performed without being affected by the cases where another signal, noise and so on exist in the blanking period such as when a back porch pulse that is inserted into a back porch as a copy guard signal exists, when a front porch period is too short due to distortion of the horizontal synchronization signal, and when large noise or the like exists.